Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to or representing sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include magnetostrictive, capacitance, ultrasonic and microwave radar, to name a few.
One form of process instrument is of the intrusive type in which the primary element is in direct contact with the process fluid for sensing level. A magnetostrictive transmitter is an example of an intrusive type level measurement instrument. A magnetostrictive transmitter has a probe including a magnetostrictive wire maintained under tension in a tube. The probe extends into the process vessel. A magnetic float is movable proximate the probe and floats atop the fluid in the vessel. An electrical pulse is transmitted on the magnetostrictive wire. The electrical pulse interacts with the magnetic field of the float, which creates a torque on the wire to produce a torsional force on the wire, thus initiating a torsional wave that propagates along the wire at the speed of sound. This is known as the Wiedemann effect. Typically, a pickup sensor is positioned at one end of the wire to sense the torsional wave on the wire. The elapsed time is measured between the launch of the electrical pulse and the signal from the pickup sensor. The distance between the magnet and the pickup sensor is calculated from the measured elapsed time multiplied by the speed of the torsional wave, representing level.
Known magnetostrictive measurement instruments use piezoelectric crystals as a pickup sensor. A known pickup sensor uses a pair of crystals mounted proximate a near end of the tube. The crystals sandwich the magnetostrictive wire so that interaction between an electric pulse on the magnetostrictive wire and the magnetic field produces the torsional wave on the magnetostrictive wire sensed by the pair of crystals. The contact force between the piezoelectric crystals and the magnetostrictive wire is critical. Ample contact force is required to couple the energy of the torsion wave to the crystal. However, excessive contact force can break the crystals. Previous designs used screws to clamp the assembly together. Contact force was controlled by the amount of torque applied to the screws. Excessive torque could cause breakage of the crystals.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.